Recently, friction stir welding has been used as a welding method of materials (Patent Literature 1: JP-A-2005-177844).
In the friction stir welding, a rotating welding tool is pressed onto to-be-joined objects to fluidize the to-be-joined objects with friction heat, resulting in jointing of the objects.
In the friction stir welding, the welding tool is attached to a spindle of a friction stir welding machine and is configured to be driven to be rotated around a rotary axis that is in a direction intersecting with surfaces of the to-be-joined objects. While being kept rotated, the welding tool is moved along a to-be-joined portion of the surfaces of the to-be-joined objects.
The friction stir welding machine is often a machine dedicated for applying pressure on the objects to generate a friction heat by axially moving the welding tool or a workpiece while rotating the welding tool with a high torque. On the other hand, a general-purpose machine tool can replace the friction stir welding machine as far as the general-purpose machine tool is used for rotating the spindle and moving the tool or the workpiece.
The welding tool is exemplified by a welding tool including a cylindrical holder and a conical pin formed on a lower end surface of the holder and extending downward and coaxially with the holder. A stepped shoulder is formed around the pin.
In an operation of the friction stir welding, the pin is pressed into the to-be-joined objects and the shoulder is kept facing the surfaces of the to-be-joined objects.
As described above, when the friction stir welding is performed with the general-purpose machine tool attached with the welding tool, since the welding tool is pressed onto the to-be-joined objects while being kept rotated, a strong force is required for pressing the welding tool into the to-be-joined objects. Accordingly, a strong thrust force is applied in a rotary axial direction on the spindle attached with the welding tool and the spindle head. Such a strong thrust force is usually unexpected in the general-purpose machine tool configured to cut with a cutting blade. Accordingly, in the machine tool including a spindle bearing (main bearing) whose thrust strength is not sufficient, the spindle bearing may possibly be damaged.
Such an excessive load applied on the spindle bearing of the machine tool is not specific to the friction stir welding, but may possibly occur in cutting among other machining methods. Accordingly, it has been desired to provide a tool holder and a machine tool which are capable of reducing the load applied on the spindle bearing.